In recent years, elimination of solvents and use of water-based constitution have been demanded in the field of recording materials and an inking material utilized for printers, printing machines, markers and writing tools. Especially, there have been widely used recording materials comprising mainly an aqueous solution of a water-soluble dye and those comprising mainly a fine particle dispersion of a pigment for a water-based ink of ink-jet recording.
In a water-based ink comprising a water-soluble dye, there is used an aqueous solution of a water-soluble dye selected from the group of dyes that are categorized mainly into acid dyes, direct dyes and some food dyes. To the aqueous solution is added a glycol or an alkanolamine as a wetting agent, a surfactant or an alcohol to adjust the surface tension, thickeners and other agents according to need. The water-based inks using water-soluble dyes are most commonly used because of their high reliability of anti-clogging at pen tips or in a printer. The water-based inks using water-soluble dyes, however, easily cause blotting on recording paper because they comprise an aqueous solution of dyes. Therefore, they have only limited uses and their print quality tends to be deteriorated due to blotting of the ink. In addition, water-soluble dyes, which only permeate into a recording paper sheet and are then dried to adhere to the recording paper sheet, is hardly considered to be “dyed”. Thus, light fastness of such water-based inks is extremely low.
Further, in order to solve problems of low water resistance and low light fastness of the water-based inks utilizing the water soluble dyes, proposals of coloring water-dispersible resins with an oil soluble dye or a hydrophobic dye have been made, for example, in JP-A No. 55-139471 (the term, JP-A refers to an unexamined and published Japanese Patent Application), JP-A No. 58-45272, JP-A No. 3-250069, JP-A No. 8-253720, JP-A No. 8-92513, JP-A No. 8-183920 and JP-A No. 2001-11347.
Proposals have been made of not only dyeing a water dispersible resin with an oil soluble dye or a hydrophobic dye but also utilizing colored particles comprising a colorant and a resin that is coated with the colorant, and also utilizing colored particles comprised of a colorant and a resin the surface of which are further coated with a film-forming resin.
On the other hand, in a pigment ink comprising mainly fine particle dispersion of a pigment, disclosed have been several proposals of coating the surface of a pigment with a film-forming resin in order to overcome problems of low density or poor color reproduction quality due to bronzing, and further, to improve light fastness, dispersion stability and discharge stability. Examples of coating a pigment with a resin are described in JP-A No. 8-269374, JP-A No. 9-151342, JP-A No. 10-88045 and JP-A No. 10-292143.
In many cases when a dispersion of first particles comprising a water dispersible resin dyed with these oil soluble dyes or hydrophobic dyes, second particles in which fine particles of a colorant such as a dye or a pigment are mixed with a resin, or third particles in which the first or second particles are further covered with another resin is prepared, a stable particle dispersion containing the colorant in a high concentration cannot be obtained due to insufficient solubility or affinity of the colorant or the resin to an organic solvent. The colorant dissolved or dispersed in a solvent is likely to precipitate, the resin is difficult to be mixed with the colorant, or the colorant is localized on the particle surface (the colorant is not covered with the resin), and therefore, ink for ink jet printing cannot be obtained which provides an image with high density. Further, in such particles, there are problems in that the effect to improve light fastness is decreased, and properties such as dispersion stability and ink jetting stability, which are needed for ink utilized for ink-jet printing.